1. Field of the Invention
The invention relates to coating compositions based on compounds containing orthoester groups and cross-linking agents, which are in particular usable as clear coats or pigmented top coats.
2. Description of Related Art
The coatings conventionally used in vehicle coating and in particular in vehicle repair coating are two-component coatings based on hydroxy-functional binder components and polyisocyanate cross-linking agents. These coatings permit the production of high quality finishes, for example, with good chemical resistance and elevated mechanical or weathering resistance. One general problem with two-component coatings, especially in repair coating, is that it is often difficult to achieve a compromise between the requirement for elevated reactivity and thus rapid drying and the requirement for an adequate pot life. Measures which bring about elevated reactivity of the coating components, for example, appropriate modifications to the binder, in general inevitably result in a reduction of the coating's pot life, (pot life=time to double in viscosity).
EP-A-882 106, for example, describes compounds containing bicyclo orthoester and spiro orthoester groups which, together with appropriate cross-linking agents, can be formulated to yield coating compositions. The orthoesters contain latent, i.e. blocked hydroxyl groups, which can be liberated by hydrolysis. Deblocking of these groups proceeds under the influence of atmospheric humidity or added water and is catalyzed. Since the orthoesters themselves are stable and the hydroxyl groups are subsequently liberated under the influence of moisture and catalysis, the intention is to achieve a good balance between pot life and drying time. However, since moisture generally gains access via atmospheric humidity, one disadvantage of these systems is that cross-linking or initiation of the cross-linking reaction is dependent on the atmospheric humidity prevailing during application. Extremely high or low atmospheric humidity during application may result in irregularities in the coatings obtained. These compounds are also relatively complicated and costly to produce.
EP-A-1 225 172 furthermore describes cross-linkable coating compositions containing polyorthoesters. These polyorthoesters are produced by reacting non-cyclic orthoesters, for example, methyl orthoformate, ethyl orthoformate, methyl orthoacetate or ethyl orthoacetate, with alpha- and/or beta-glycols and further compounds comprising at least two hydroxy groups. These polyorthoester compounds have all of the hydroxyl groups blocked and the groups are deblocked under the influence of moisture with acidic catalysis, which in turn makes the cross-linking reaction dependent on ambient conditions during application of the coating composition, in particular, on relative atmospheric humidity. Moreover, coating compositions based on these polyorthoesters and based on polyisocyanates have a poor drying performance.
There is accordingly a requirement in vehicle repair coating for coating compositions based on hydroxy components and cross-linking components reactive therewith, which exhibit a good balance between pot life and reactivity or drying time and the cross-linking reaction of which is relatively independent of ambient/climatic conditions during application of the coating composition, in particular independent of relative atmospheric humidity. The coating compositions should furthermore exhibit an elevated solids content at the viscosity required for application, in particular spray application.